Triglycerides can be described, for example, by the following formula:
    in which    R1 to R3=C10 to C30-alkyl.
These triglycerides are, for example, an important constituent of synthetic and natural fats or vegetable oils, for example palm oil, sunflower oil, soy oil or rapeseed oil, which is used in biodiesel production. The triglycerides used in accordance with the invention may be present in contaminated form or in the form of mixtures.
Fatty acid alkyl esters or biodiesel are described in the present context by the following formula:
in which R=R1 to R3 and R′=C1 to C10-alkyl or C3- to C6-cycloalkyl.
Mixtures of these fatty acid alkyl esters are the main constituent of biodiesel. R′ is preferably a CH3 or C2H5 group, but usually a CH3 group, since methylates in methanolic solution are usually used in biodiesel production. The transesterification of the triglycerides can also be performed with other alcohols to give corresponding fatty acid esters.
An overview of the state of use of vegetable oils and further fats for preparing biodiesel is described in G. Knothe, J. Van Gerpen, J. Krahl, The Biodiesel Handbook, OACS Verlag, 2005.
In general, the transesterification of triglycerides to fatty acid alkyl esters can be accelerated by acidic or basic catalysis. In industry, predominantly the more rapid homogeneous base catalysis is used. Preference is given to using sodium methoxide or potassium methoxide.
In the industrial production of biodiesel, important process parameters for the optimization of the yield include the concentration of the catalyst, the temperature, the residence time, the moisture content, the presence of free fatty acids and the alcohol excess (B. Freedman, E. H. Pryde, T. L. Mounts, Variables affecting the yield of fatty esters from transesterified vegetable oils, J. Am. Oil Chem. Soc., 61, 1638, 1984).
The triglycerides used contain about 10% by weight of glycerol, which is preferably isolated from the biodiesel fraction as a contaminated fraction by a phase separation or extraction after the transesterification. Both fractions are then very substantially freed of water, acids, catalysts, alcohol, salts and by-products in multistage workup steps.
Acrolein is an important intermediate and is thus of great economic significance for the preparation of acrylic acid, D,L-methionine and the methioninehydroxy analogue (MHA,=2-hydroxy-4-methylthiobutyric acid). Methionine is an essential amino acid which is used, inter alia, as a supplement in feeds. Nutrition-improving feed additives are nowadays an indispensable constituent of animal nutrition. They serve for better utilization of the food available, stimulate growth and promote protein formation. One of the most important of these additives is the essential amino acid methionine, which assumes a prominent position as a feed additive in poultry breeding in particular. In this field, so-called methionine replacements such as the methionine hydroxy analogue have, though, gained not inconsiderable significance, since they have similar growth-stimulating properties to the amino acid known for this purpose.
According to the background art, acrolein is synthesized by heterogeneously catalyzed selective oxidation of propene over mixed oxide catalysts. EP 417723 describes the synthesis of complex mixed multimetal oxide catalysts at temperatures of 300 to 380° C. and pressures of 1.4 to 2.2 bar. Ullmann's Encyclopaedia of Industrial Chemistry, 6th edition, 1999 describes the overall process including workup, in which a plurality of by-products are removed. Once the reactant mixture composed of propene, air and water has been converted at least partly over the catalyst, quenching to remove high-boiling by-products such as polymers, acrylic acid and acetic acid is effected first. In the downstream absorber, acrolein is extracted by washing. After the desorption, to recover the absorbent, the crude acrolein obtained is purified by distillation in several stages.
Scientific studies of the synthesis of acrolein from isolated glycerol are known. It is also known, for example, that glycerol can be dehydrated in the presence of acidic substances to various products.
According to Organic Synthesis I, 15-18 (1964), treatment of a mixture of pulverulent potassium hydrogensulphate, potassium sulphate and glycerol at 190 to 200° C. affords acrolein in a yield between 33 and 48%. Owing to the low yields and the high salt burdens, this process is, however, unsuitable for the industrial scale.
In the course of the studies of model substances of biomass pyrolysis oils, the catalytic treatment of glycerol over H-ZSM5 zeolites at 350 to 500° C. has also been studied—see Dao, Le H. et al. ACS Symp. Ser.: 376 (Pyrolysis Oils Biomass) 328-341 (1988). Hydrocarbons are formed only in low yields, but the formation of acrolein is pointed out.
WO 2006/092272 discloses a process for preparing acrylic acid or acrylic acid polymers by dehydrating glycerol to a dehydration product comprising acrolein, gas phase oxidation and subsequent isolation of acrylic acid and subsequent polymerization. However, a process for controlled acrolein preparation is not described.
DE 42 38 493 describes the acid-catalyzed conversion of glycerol to acrolein in the gas phase and in the liquid phase. DE 42 38 492 further relates to the synthesis of 1,2- and 1,3-propanediol by dehydrating glycerol with high yields. The glycerol used is usually in pure form or in the form of an aqueous solution.
The glycerol obtained after the phase separation in the biodiesel production is, however, generally of low value, since it is highly contaminated, for example, by excess methanol, catalyst and soaps.
To date, the above-described steps have never been combined in order to use triglycerides directly for the simultaneous preparation of fatty acid alkyl esters, especially of biodiesel and acrolein.
On the one hand, the separate preparation of biodiesel from triglycerides and acrolein from propene or glycerol gives rise to a comparatively high level of apparatus complexity and hence corresponding capital costs, since synergies in the combined preparation in an integrated system are not utilized. What also arises is corresponding logistical complexity and hence correspondingly high variable costs, for example transport and energy costs, in order to convey glycerol.
On the other hand, the disadvantage of the classical acrolein production to date, by selective oxidation from propene, is considered especially to be the complicated process in which propene has to be prepared in the gas phase and has to be isolated in the multistage workup, and also that propene is a comparatively expensive starting material whose costs are additionally increasing in a greater-than-proportional manner at the current time.